Disclosed in JP-A-2007-154684 is a two-stage supercharged vehicle engine for which a high-pressure stage turbocharger and a low-pressure stage turbocharger are arranged in series. In general, a compressor used with a turbocharger has such characteristics that its efficiency varies with an air (volume) flow rate and a pressure ratio. The characteristics are expressed in the form of a so-called compressor map. A two-stage supercharged engine can use its exhaust energy more effectively by using a small-capacity compressor exhibiting high-efficiency at a low flow late for the high-pressure stage turbocharger and using a large-capacity compressor exhibiting high-efficiency at a high flow rate for the low-pressure stage turbocharger. In other words, while the engine is conducting a low-speed, low-load operation, the high-pressure stage turbocharger exhibiting high efficiency at a low flow rate is operated with the low-pressure stage turbocharger substantially stopped because an intake air amount is small. On the other hand, while the engine is conducting a high-speed, high-load operation, an intake air amount adequate for an increase in engine power is obtained by operating only the low-pressure stage turbocharger having a large capacity in such a manner as to let an engine exhaust gas bypass a high-pressure stage turbine of the high-pressure stage turbocharger. A high boost pressure can be obtained over a wide operating range by selectively using either the high-pressure stage turbocharger or low-pressure stage turbocharger in accordance with engine speed and load as described above.    Patent Document 1: JP-A-2007-154684    Patent Document 2: JP-A-2005-83285    Patent Document 3: JP-A-1998-176558    Patent Document 4: JP-A-1999-324746